Habitat use and spatio-temporal interactions of mule and white-tailed deer in an area of sympatry in NE Washington

Sympatric species that are ecologically similar must either segregate through habitat disassociation or engage in biotic interactions with one another. Mule (Odocoileus hemionus) and white-tailed deer (Odocoileus virginianus) are similar ungulate species that are distributed across North America in both areas of sympatry and allopatry. Over many decades, white-tailed deer have been expanding their range into areas historically used allopatrically by mule deer, potentially leading to increased interactions between the species. However, the degree to which the two species segregate spatio-temporally or engage in agonistic interactions is yet unclear. Therefore, to compare their realized habitat niches in an area of sympatry, we determined presence and absence of both deer species at 312 camera traps during the summers of 2018-2019 across a range of elevations and habitats in the Colville National Forest (CNF) in northeastern Washington. We compared characteristics of habitats used by the two species using single-species occupancy models and found that topography was the strongest predictor of differing habitat use. Mule deer were more likely to occupy steep slopes and higher elevations and white-tailed deer more likely to occupy shallower slopes and lower elevations. Using conditional, two-species occupancy modeling, we found that after accounting for differences in habitat selection between the species, mule and white-tailed deer occurred independently of one another in the CNF during summer. We found no evidence for temporal segregation, with similar to 90% overlap in daily activity patterns and similar elapsed time between subsequent intra- and interspecies detections at the 21% of camera sites where both species were detected. Niche segregation along topographic gradients in our study system likely reduces the potential for current interspecies interactions, positive or negative, to occur in this system. However, we did not find any evidence of ongoing spatio-temporal avoidance, even in areas of the landscape where both species were detected (e.g., sharing habitat). Therefore, although our observational study cannot rule out that contemporaneous habitat segregation is evidence of past competitive interactions between the species, our research does not strongly support the contention that agonistic interactions are currently occurring between mule and white-tailed deer.